The present invention relates to a slip-controlled brake system for motor vehicles with four-wheel drive, wherein one axle is driven permanently, and a second axle is driven by a differential-slip-sensitive coupling. Sensors are provided for determining the rotational behavior of the wheels. Electronic switching circuits are provided for the logical processing of sensor signals and for the generation of brake pressure control signals as a function of the rotational behavior of the wheels. An auxiliary-pressure supply system is provided in which, during the traction slip control phase, brake pressure is introduceable into the wheel brakes of the slip-controlled wheels.
Brake systems of this type are known wherein, for the limitation and control of the traction slip, brake pressure from an auxiliary-pressure supply system is introduceable in a dosed manner by way of electromagnetically actuatable multi-directional control valves into the wheel brakes of the wheels having an excessive slip (German published patent application No. 33 27 401). Systems of this type are combined with anti-lock systems or designed as an additional apparatus to anti-lock brake systems. This is done because most of the components, e.g. the wheel sensors, the auxiliary-pressure supply system and some of the electronic switching circuits, are required both for the brake slip and for the traction slip control. Therefore, the additional expenditure required for a combined system is, compared with the expenditure for pure anti-lock or traction slip control system, relatively low.
When designing a traction slip control system for vehicles with four-wheel drive, one would expect that either a connection of all wheel brakes to the traction slip control system or the installation of differential locks were necessary. Both measures would require a high expenditure and would have technical disadvantages. Since, normally, the brake pressure in the rear wheels is controlled in phase according to the select-low principle, and both rear wheel brakes are connected to the same brake circuit, the wheel brakes of both rear wheels would have to be separated hydraulically and electronically in order to achieve an individual traction slip control. Due to, among other things, the rigid coupling of the wheels by way of the drive trains the insertion of differential locks leads to considerable difficulties when determining the vehicle reference speed required for the control action.
It is, therefore, an object of the present invention to develop a traction slip-controlling brake system which is suitable for vehicles with four-wheel drive and a differential-slip-sensitive coupling between the front wheel drive and the rear wheel drive, and which can be manufactured at minimum expenditure.